1. Field of the Invention
This invention relates to a method for reading an optical information medium formed with record marks of the size smaller than diffraction limit, and an optical information medium adapted for use in such method.
2. Prior Art
Various types of optical information media are known in the art including read only optical discs such as compact discs, rewritable optical recording discs such as magneto-optical recording discs and phase change optical recording discs, and write-once optical recording discs such as those utilizing organic dyes for the recording material.
In the case of an optical information medium, information density can be generally increased to a level higher than magnetic recording media. However, further increase in the information density is required in view of the enormous amounts of information that should be handled in the case of image data and the like. Typical methods used for increasing the information density per unit area are reduction of the track pitch and increase of linear density by reducing the distance between the record marks or phase pits. However, excessive increase in the track density and the linear density in relation to the beam spot of the reading light is associated with unduly reduced CNR (carrier to noise ratio), and the signals eventually becomes unreadable. The resolution in the reading of signals is determined by the diameter of the beam spot, and more specifically, the read limit is generally spatial frequency 2NA/xcex when the reading light has a wavelength of xcex and the optical system of the reading system has the numerical aperture, NA. Accordingly, reduction in the wavelength of the reading light and increase in NA are effective in improving the CNR and the resolution upon reading. Although many technical investigations are underway, various technical problems are to be solved before introduction of such techniques.
In view of such situation, various methods for surpassing the read limit determined by the light diffraction, namely, the so-called super-resolution readout methods have been proposed.
The super-resolution readout method most typically used is the method wherein a mask layer is disposed on the recording layer. In this method, an optical aperture smaller than the beam spot is formed in the mask layer by utilizing the fact that the intensity of the laser beam spot is in Gaussian distribution to thereby reduce the size of the beam spot to a size smaller than the diffraction limit. Such method is roughly categorized by the difference in the mechanism of optical aperture formation into heat mode system and photon mode system.
In the heat mode system, the area in the mask layer within the beam spot that has reached the predetermined temperature undergoes change in optical properties. Such heat mode system is used, for example, in the optical disc described in Japanese Patent Application Laid-Open No. (JP-A) 205314/1993. This optical disc has a transparent substrate having optically readable record pits formed thereon according to the information signals, and a layer disposed thereon comprising a material that experiences change in reflectivity by the temperature. This layer functions as the mask layer. The materials specifically named in JP-A 5-205314 are lanthanoids. In the optical disc described in JP-A 5-205314, reflectivity of the layer formed of the material as described above changes corresponding to the temperature distribution within the scanning spot of the readout beam, and the reflectivity returns to the initial level after the completion of the readout by the temperature decrease and the layer does not melt in the reading operation. The heat-mode super-resolution is not limited to the one as described above, and also known is the system as disclosed in Japanese Patent No. 2844824 wherein a material which undergoes amorphous-crystalline transition is used for the mask layer so that the high-temperature area within the beam spot undergoes phase transition to become crystalline and to exhibit improved reflectivity whereby super-resolution readout is enabled.
In the heat mode super-resolution, the size of the optical aperture is uniquely determined by the temperature distribution of the mask layer. Accordingly, the optimal power of the reading light should be determined by taking all of the linear velocity of the medium, the power of the reading light, the temperature of the atmosphere surrounding the medium during the readout operation into consideration.
In the case of the photon-mode super-resolution, change in the optical properties occurs in the area wherein amount of the photon has reached the predetermined value. The photon-mode super-resolution is utilized, for example, in the information recording medium described in JP-A 96412/1996, in the optical recording medium described in JP-A 86342/1999, and in the optical information recording medium described in JP-A 340482/1998. JP-A 96412/1996 discloses a mask layer comprising a resin having a phthalocyanine or its derivative dispersed therein and a mask layer comprising a chalcogenide. JP-A 86342/1999 discloses use of a mask layer comprising a super-resolution readout film comprising a semiconductor material having a photonic band gap at which the light absorption properties undergo a change by excitation of the electron to the energy level of the exciton by the irradiation of reading light. Exemplary mask layer disclosed therein is a mask layer comprising a matrix of SiO2 having CdSe fine particles dispersed therein. JP-A 340482/1998 discloses use of a mask layer comprising a glass layer wherein the intensity distribution of the light that has transmitted through the mask layer varies in a non-linear relationship with the intensity distribution of the light irradiated.
In the case of a photon-mode super-resolution readout medium, the medium is not influenced by the temperature surrounding the medium during the reading operation in contrast to the heat-mode super-resolution readout medium, and the medium is relatively resistant to deterioration by repeated read operations.
In the case of a photon-mode super-resolution, the area which undergoes change in optical properties is determined by the number of photons that has entered the area, and such number depends on the linear velocity of the medium in relation to the beam spot. The size of the optical aperture also depends on the power of the reading light in the photon mode super-resolution, and application of an excessive power results in the formation of an excessively large optical aperture and the super-resolution readout is no longer possible. In other words, determination of the optimal power for the reading light is also necessary in the case of photon-mode super resolution depending on the linear velocity and the size of the pit and record mark of the medium to be read.
Determination of optimal read power is necessary in the super-resolution readout as described above irrespective of whether the readout is conducted by heat-mode super-resolution or photon-mode super-resolution.
Various proposals have been made for the determination of the optimal read conditions and an exemplary such proposal is provision of a test area for test read of the optical information medium. For example, JP-A 126340/1999 discloses a method wherein a plurality of test areas are provided at different radial positions for the determination of the optimal read power which varies by the operating temperature and the distance from the center. In JP-A 126340/1999, test read is conducted by altering read power in each test area, and the optimal read power is determined from the error rate of the readout signals. JP-A 126340/1999 also discloses that the evaluation of the readout signals may be conducted on the bases of signal amplification. In the procedure specifically described in JP-A 126340/1999, optimal read power is determined in a magneto-optical disc adapted for MSR (Magnetically induced Super Resolution) technology by reading a test pattern of single signal comprising record marks and the spaces therebetween each having a length 0.38 xcexcm. JP-A 126340/1999, however, is silent about whether the record mark having a length of 0.38 xcexcm is of a size beyond the diffraction limit.
However, incorporation of an error rate-detection circuit in a commercially available optical drive is impractical in economical point of view. Determination of the optimal read power by the use of absolute value of the amplitude of the signal that has been read out is also associated with difficulty since signal processing system of an optical disc drive has AGC (automatic gain control) function incorporated therein for the control of the level of the readout signal.
In view of the situation as described above, an object of the present invention is to provide a method for determining optimal read power for the optical information medium adapted for high resolution readout beyond diffraction limit. Another object of the present invention is to provide an optical information medium used in such method.
The objects as described above are attained by the (1) to (3), below.
(1) A method for reading an optical information medium wherein readout is conducted under the conditions wherein the relation:
LMIN less than xcex/4NA
is met when reading light has a wavelength xcex, objective lens of readout optical system has a numerical aperture NA, and minimum mark of the marks constituting the information pattern has a length LMIN; wherein
a short mark having a length LS and a long mark having a length LL meeting the relations:
LS less than xcex/4NA, and
LL≅xcex/4NA
are preliminarily formed in the optical information medium, and the optimal power of the reading light is determined on the output of said short mark and the output of said long mark are read.
(2) A method for reading an optical information medium according to the above (1) wherein said optical information medium has a data recording area and a test read area, and said short mark and said long mark are preliminarily formed in said test read area.
(3) An optical information medium wherein readout is conducted under the conditions wherein the relation:
xe2x80x83LMIN less than xcex/4NA
is met when reading light has a wavelength xcex, objective lens of readout optical system has a numerical aperture NA, and minimum mark of the marks constituting the information pattern has a length LMIN, and
wherein said optical information medium has a data recording area and a test read area and a short mark having a length LS and a long mark having a length LL meeting the relations:
LS less than xcex/4NA, and
LL≅xcex/4NA
are preliminarily formed in said test area.